The present invention relates generally to the fabrication of alkaline electrochemical cells and, in particular, relates to the preparation of an anode having a mercury-free zinc-based anode.
Alkaline electrochemical cells such as metal-air batteries produce electricity by electrochemically coupling in a cell a reactive metallic anode to an air cathode through a suitable electrolyte. As is well known in the art, an air cathode is typically a sheet like member having opposite surfaces that are exposed to the atmosphere and to an aqueous electrolyte of the cell, respectively. During operation, oxygen from air dissociates at the cathode while metal of the anode oxidizes, thereby providing a usable electric current flow through the external circuit between the anode and the cathode.
Most metallic anodes are thermodynamically unstable in an aqueous neutral or alkaline electrolyte and, react with the electrolyte to corrode or oxidize the metal and generate hydrogen gas. This corrosive shelf-discharge side reaction can reduce both service and shelf life of alkaline electrochemical systems that use zinc as the anode active material. During discharge, electrochemical oxidation occurs at the anode, and metallic zinc is oxidized to zinc hydroxide, zincate ions, or zinc oxide. Under conditions such as high discharge rates or low alkaline electrolyte concentration, where the product of discharge is too densely attached to the surface, passivation of the zinc can occur. The presence of a solid phase zinc oxide or hydroxide film will interfere with the discharge efficiency of the zinc anode.
Mercury has traditionally been added to the zinc anode to improve the corrosion and discharge behavior of the anode. However, because mercury has been shown to be environmentally unfriendly, it has become more desirable to produce mercury-free anodes. Among various technologies for avoiding mercury, the most advanced in the prior art is to add a small amount of metal, such as lead, calcium, indium, bismuth, and combinations thereof, to the zinc to provide a zinc alloy. Unfortunately, it has been discovered that alkaline electrochemical cells using mercury-free zinc alloy containing such additives (mercury-free zinc, in particular) exhibit a significant drop in both operating voltage and service life when compared to zinc anodes having a mercury additive, especially when the cell discharged at a high rate. This is mainly attributed to either zinc particle surface passivation, caused by zinc oxide forming at the zinc surface, and/or anode polarization. These may both be caused by the lack of a sufficient quantity of hydroxyl ions in the anode, and/or a sufficiently even distribution of hydroxyl ions.
What is therefore needed is an environmentally friendly zinc-based anode having an extended service life and higher operating voltage than presently achieved.